1. Field of the Invention
The present invention relates to an effective-value converting circuit adapted to convert an a-c input voltage or current into a d-c signal proportional to the effective value of the input.
2. Description of the Prior Art
For measurement of an a-c voltage or current, normally available is a rectifier type instrument which rectifies an a-c input by a rectifier and then feeds it to a moving-coil type meter to indicate the input value. In such a rectifier type instrument, it is possible to achieve a high sensitivity and a high accuracy since the principle of operation is the same as that of the moving-type meter. By virtue of this merit, the rectifier type instrument is widely employed for measuring a-c voltage or current. When a sine-wave a-c input having an effective value Iin is fed to this instrument, its pointer indicates the mean value (2.sqroot.2/.pi.)Iin of the input current Iin. In general, as the a-c value used practically is the effective one, it is ordinary that the scale of the rectifier type instrument is graduated to show a value obtained through multiplying the intrinsic indication by a form factor (.pi./2.sqroot.2) so that the sine-wave effective value Iin is directly readable. Due to such principle of operation, the form factor fails to become .pi.2.sqroot.2 in the rectifier type instrument when there is any distortion in the a-c input waveform to be measured, hence causing an indication error. In order to attain accurate measurement of the a-c input without undesired influence of distortion, an effective-value indicating type instrument is necessary. To meet this requirement, there is available a moving-iron type meter which is capable of directly indicating the effective value of the a-c input to be measured, thereby causing no waveform error unlike in the rectifier type instrument. On the other hand, while the moving-iron type meter features the above-described merit, an extremely high sensitivity is not achievable as viewed from its structure. Furthermore, it has another disadvantage that an indication error may occur depending on the frequency of an input to be measured. Thus, there exist some demerits in both the rectifier type and moving-iron type instruments serving as a-c measuring means.
Recently an improved effective-value converting circuit has become available, in which an a-c input to be measured is rectified by a rectifier circuit and then is fed to a nonlinear circuit consisting of resistors and diodes and a capacitor so as to obtain a d-c output signal proportional to the effective value of the a-c input. By connecting this effective-value converting circuit with a moving-coil type meter to indicate an a-c input, it is possible to accomplish a high-sensitivity, high-accuracy instrument free from waveform error, that is, an improved a-c measuring instrument having advantages of both the conventional rectifier type and moving-coil type instruments. However, in the effective-value converting circuit employing a nonlinear circuit, the d-c output signal obtained in the case of a low input is not satisfactory in its linearity, as will be described afterward. Accordingly, in constituting an a-c measuring instrument through combination of the above effective-value converting circuit with a moving-coil type meter, there arises a problem that its scale graduations fail to be uniform. Concerning the unsatisfactory linearity, the condition is the same as in the conventional rectifier type instrument or even worse particularly in the moving-iron type instrument. Although the rectifier type or moving-iron type instrument has various demerits as already mentioned, problematical points are minimum in the a-c instrument using an effective-value converting circuit with a nonlinear circuit. Therefore, on condition that the problem of linearity is solved the instrument equipped with an effective-value converting circuit is considered to be ideal for measurement of a-c voltage or current.
The principal object of the present invention is, in an effective-value converting circuit employing a nonlinear circuit, to provide an improvement having excellent linearity by reforming the linearity of a d-c output with respect to an a-c input.
Another object of the invention resides in providing an improved effective-value converting circuit having uniform scale graduations in an instrument constituted through combination of a moving-coil type meter with an effective-value converting circuit including a nonlinear circuit.
And a further object of the invention is, in an instrument composed of a moving-coil type meter with an effective-value converting circuit including a nonlinear circuit, to provide an improved converting circuit having such characteristics that the scale graduations are uniform up to a certain input value and are suppressed for an input exceeding the said certain value.